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The woman is young, perhaps 18, with olive skin and dark bangs that droop onto her face. In the Facebook photo, she attempts to smile but doesn't look at her photographer.

The caption mentions a single biographical fact: She is for sale.

"To all the bros thinking about buying a slave, this one is US$8000," begins the May 20 Facebook posting, which was attributed to an Isis (Islamic State) fighter who calls himself Abu Assad Almani. The same man posted a second image a few hours later, this one a pale young face with weepy red eyes.

"Another sabiyah [slave], also about US$8000," the posting reads. "Yay, or nay?"

The photos were taken down within hours by Facebook, and it is unclear whether the account's owner was doing the selling himself or commenting about women being sold by other fighters. But the unusual posting underscores what experts say is an increasingly perilous existence for the hundreds of women who are thought to be held as sex slaves by Isis.

As the terrorist group comes under heightened pressure in Iraq and Syria, these female captives appear to be suffering, too - sold and traded by cash-strapped fighters, subjected to shortages of food and medicine, and put at risk daily by military strikes, according to terrorism experts and human rights groups.

Social-media sites used by Isis fighters in recent months have included numerous accounts of the buying and selling of sex slaves, as well the promulgation of formal rules for dealing with them. The guidelines cover such topics as whether it's possible to have sex with prepubescent prisoners - yes, Isis' legal experts say - and how severely a slave can be beaten.

But until the May 20 incident, there were no known instances of Isis fighters posting photographs of female captives being offered for sale. The photos of the two unidentified women appeared only briefly before being deleted by Facebook, but the images were captured by the Middle East Media Research Institute, a Washington nonprofit group that monitors jihadists' social-media accounts.

"We have seen a great deal of brutality, but the content that Isis has been disseminating over the past two years has surpassed it all for sheer evil," said Steven Stalinsky, a researcher for the institute. "Sales of slave girls on social media is just one more example of this."

Almani, the apparent owner of the Facebook account, is thought to be a German national fighting for Isis in Syria, according to Stalinsky. He has previously posted to social-media accounts under that name, in the slangy, poorly rendered English used by many European fighters who can't speak Arabic. Early postings suggest that Almani is intimately familiar with Isis' activities around Raqqa, the group's de facto capital in Syria. He also regularly uses his accounts to solicit donations for the terrorist group.

In displaying the images of the women, Almani advised his Facebook friends to "get married" and "come to dawlah," or Isis' territory in Iraq and Syria. Then he engaged with different commenters in an extensive discussion about whether the US$8000 asking price was a good value. Some who replied to the postings mocked the women's looks, while others scolded Almani for posting photos of women who weren't wearing the veil.

"What makes her worth that price? Does she have an exceptional skill?" one of his correspondents asks about woman in the second photo.

"Nope," he replies. "Supply and demand makes her that price."

Isis' leaders have historically used US-based social media such as Facebook and Twitter to attract recruits and spread propaganda, but in the past year American companies have sought to block jihadist accounts and postings whenever they are discovered.

Facebook in particular has garnered high marks from watchdog groups for reacting quickly to terrorists' efforts to use its pages. But at the same time, the militants also have become more agile, leaping quickly from one social-media platform to another and opening new accounts as soon as older ones are shut down.

The Facebook incident comes amid complaints from human rights groups about waning public interest in the plight of women held as prisoners by Isis. Human Rights Watch, citing estimates by Kurdish officials in Iraq and Syria, says the terrorist group holds about 1800 women and girls, just from the capture of Yazidi towns in the region. After initial denials, Isis last year issued statements acknowledging the use of sex slaves and defending the practice as consistent with ancient Islamic traditions, provided that the women are non-Muslims captured in battle or members of Muslim sects that the terrorist group regards as apostates.

A report last month by Human Rights Watch recounted the ordeals suffered by three dozen Iraqi and Syrian women who escaped from terrorist-held towns in recent months. Among the women were former Yazidi sex slaves who described abuses that included multiple rapes by different men as they were sold and traded.

The problems faced by such women appear to be growing worse as military and economic pressure against Isis increases, the report said.

"The longer they are held by Isis, the more horrific life becomes for Yazidi women, bought and sold, brutally raped, their children torn from them," said Skye Wheeler, women's rights emergencies researcher at Human Rights Watch. "Meanwhile, Isis' restrictions on [non-enslaved] Sunni women cut them off from normal life and services almost entirely."